What is your approach to differentiating diabetes insipidus from primary polydipsia in the outpatient setting?  

I usually do overnight dehydration tests for 12 hours and if fasting AM urine osmolality is >600 DI is less likely. If urine osmolality is low with high serum sodium it indicates DI, whereas with primary polydipsia the serum sodium is low with low urine osmolality.